Switching system



Feb. 9, 1943. A. MEACHAM ETAL 2,310,452

SWITCHING SYSTEM 7 Filed June 5, 1941 6 Sheets-Sheet l F IG 5 FIG. 7

FIG 4 FIG 5 FIG 2 FIG. .9

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A TTORNEY b- 1943- A. MEACHAM ETAL 310.452

SWITCHING SYSTEM Filed June 5. 1941 6 Sheets-Sheet 2 LAMEACHAMm/vE/vmRs. C M PARKER By WW K A T TOR/V5 V Feb. 9, 1943. L. A. MEACHAMETAL SWITCHING SYSTEM Filed June 5, 1941 6 Sheets-Sheet 3 /Nl/ENTORS;

By I T: 1

ATTORNEY NE a Ref. 9, 1943. A. MEACHAM EI'AL SWITCHING SYSTEM Filed June5, 1941 v6 Sheets-Sheet 4 Q MIVENTORS.

C l/ PARKER A T TOP/V5 V Feb; 9, 1943. 1.. A. MEACHAM El'AL 2,310,452

SWITCHING SYSTEM Filed June 5, 1941 6 Sheets-Sheet 5 L. A. MEACHAMWVENTORS. Gym/"(ER WWW A 7' TORNE Y Feb. 9, 1943. A. MEACHAM ETAL2,310,452

SWITCHING SYSTEM Filed June 5, 1941 6 Sheets-Sheet 6 IMPUL .S'E GENERATOR LAMEA M mn/E/vroRs. CVPARKER WWWKW A T TORNEV ?tente Feb 1943SWITCHING 'SYSTEM Larned A. Meacliam, Summit, N. 3., and Carlyle V.Parker, Washington, D. 0., assignors to Bell Telephone Laboratories,Incorporated, New

York, N. Y., a corporation of New York Application June 5, 1941, SerialNo. 396,687

7 Claims.

This invention relates to switching systems and particularly to suchsystems when used for establishing telephone or other communicationconnections.

The objects of the invention are to simplify these switching systems, tominimize the use of common controlling equipment, to increase the speedof operation, and to obtain other improvements therein.

In automatic telephone systems where large groups of lines have accessthrough switches, such as line finders or line switches, to trunk groupsit has been proposed heretofore to arrange these switches in primary andsecondary formation with links for extending the lines to the trunks andalso to arrange the switches and lines in coordinate rows or groups,such as horizontal and vertical groups, for the purpose of obtaining anorderly interconnection of the lines with the trunks. In such aswitching system, and particularly when crossbar switches are used, ithas usually been necessary to provide relay chains and rather involvedcontrol circuits for serving the lines having calls thereon, forpreventing interference between simultaneous calls in the differentgroups, for insurin selection in preferred horizontal and verticalgroups, for selecting idle groups of trunks, and for selecting idlelinks having access to these trunk groups. Also in these prior systemsit has been necessary to provide each group of primary and secondaryswitches, such as those occupying a, frame, with a considerable amountof equipment that is used in common by all switches on the frame.

Accordingly a feature of the present invention is a system in which therelay selecting and testing mechanisms are replaced by electronicdischarge devices which perform the testing, selecting, and othercontrolling functions with greater speed, and in which much of thecommon controlling circuits and equipment have been eliminated. Morespecifically a calling line in one of the primary switches is extendedover a link to a secondary switch and thence to an idle trunk in thegroup accessible to such secondary switch by selectively operating withimpulses of difi'erent phases one of a series of discharge tubes toselect the horizontal group of primary switches containing the callingline; by utilizing this tube to apply potential to the switch magnetoperating circuits of all links extending from the chosen horizontalgroup to the respective secondary switches, each operating circuitincluding the operating magnets associated with a given link on allprimary switches in the group and the magnet of the secondary switch towhich the corresponding link extends; by operating by means of phaseimpulses one of a series of tubes representing a trunk group having idletrunks therein and an idle link extending from the calling horizontalgroup of primary witches to the secondary switch having access to suchtrunk group; and by utilizing the operated tube to apply potential atthe chosen secondary switch to the magnet operating circuits of alllinks extending to said secondary switch from each of the severalprimary switch groups. Thus at the primary switches the magnet operatingcircuits are prepared for all links extending from the chosen horizontalgroup to the several secondary switches, and at the chosen secondaryswitch the magnet operating circuits are prepared for all linksextending thereto from the several primary horizontal groups; and theresult is that one and only one of these circuits is completed to effectthe desired connection, the choice of the idle link and trunk group fromother links and groups, which may be busy or idle at the time, beingmade by applying phase impulses to the selecting tubes.

Another feature of the invention is a system of this kind in which theextension of a calling line to an idle trunk results in the discharge ofa tube common to the trunk group to cause the release of the operatedtubes and circuits associated with the frame, whereby the switches andlinks of the frame become available to other calling lines.

These and other features of the invention will be described more fullyin the following detailed specification.

In the drawings accompanying the specification:

Fig, 1 is a diagram of a frame of primary and secondary switches,illustrating in particular the energizing circuits of the switchoperating magnets;

Figs. 2 to 6 when arranged as indicated in Fig. 7 disclose the detailedcircuits and equipment for a portion of one of the line switch framesillustrated in Fig. 1;

Fig. 2 shows a horizontal row or group of primary line switches;

Fig. 3 illustrates another horizontal row of primary switches and alsoshows the horizontal group selecting equipment for choosing thehorizontal group to be served;

Figs. 4 and 5 show two of the secondary line switches to which the linksincoming from the several horizontal primary groups have access andwhich have access to respective outgoing groups of trunks; and- Fig. 6shows the common controlling mechanism for performing the link and trunktesting operations and for releasing the circuits after use.

Referring first to Fig. 1, the frame of crossbar line switchesillustrated therein comprises ten horizontal groups or rows of primaryline switches, of which two are shown, the No. group I00 and the No. 9group IOI. Each horizontal group may have any desired number of primaryswitches therein, it being assumed in the present case that sevenprimary switches are provided in each of the ten horizontal groups. Twoof the seven primary switches I02 and I03 are shown in the horizontalgroup I00, and two primary switches I04 and I05 are shown in the groupIOI. Each of these primary line switches serves ten subscribers lines,the lines appearing in the vertical rows of contacts of the switches.For example, lines I06 and I0I appear in the primary line switch I02,and lines I08 and I09 appear in the primary line switch I03.

The line links which interconnect the primary line switches and'thesecondary line switches appear in the horizontal rows of contacts in theswitches of the difierent primary horizontal groups. There are ten ofthese links for each horizontal group, and all ten links are multipledto appear in the horizontal rows of contacts or all of the seven primaryswitches.

Ten secondary switches are also provided on the line switch frame, therebeing one of these secondary switches for each horizontal group oiprimary switches. switches are shown, the No. 0 switch H0 and the No. 9switch III. The primary and secondary switches are interconnected by theline links in such a manner that all ten primary groups have access toall ten secondary switches.- This is accomplished by extending the tenlin links from each horizontal group respectively to the ten secondaryswitches. Stated in another way, each secondary switch receives a singlelink from each of the ten primary groups.

The ten secondary switches have access respectively to ten outgoinggroups of trunks, to which the calling lines are connected through thejoint operation of the primary and secondary switches. For example, thetrunk group I I2 appears in and is accessible to the secondary lineswitch I I0, and the trunk group I I3 appears in the secondary lineswitch III. Eight other trunk groups appear respectively in theremaining eight secondary switches not shown in the drawings.

Each of the primary and secondary switches on the frame is provided withten select magnets, one for each horizontal row of contacts, forpreparing the switch for operation and with ten hold magnets individualrespectively to the vertical rows of contacts for holding the contactsin their operated condition after the select magnets have been released.For example, the select magnets I I4 and I I5 are individualrespectively to the No. 0 and No. 9 horizontal rows in the switch I02.Similarly the select magnets H6 and II! serve the No. 0 and No. 9horizontal rows of contacts in the switch I03. The two line link H8 andH9 of the horizontal group I20 appear respectively in the No. 0 and No.9 horizontal rows of switches I02, I03, etc., of the horizontal groupI00. Similarly the select magnets I2I, I22 of switch I04, and I23, I24of switch I05 control the No. 0 and No. 9 horizontal rows of contacts inthese switches, in which the line links I25 and I26 of the group I2!appear. The links of the Two of these secondary group I20 appearrespectively in the No. 0 horizontal rows of contacts in the tensecondary switches. the link H8 in the No. 0 row of switch H0 and thelink H9 in the No. 0 row of switch III. These horizontal 'rows inswitches H0 and III are controlled by the selectmagnets I28 and I29. Thelinks of the group I2I appear in the No. 9 horizontal rows of contactsin the respective secondary switches and are controlled by the selectmagnets thereof, such as the magnets I30 and I3I of switches H0 and III,respectively.

As will be described more fully hereinafter, the select magnets of theprimary and secondary switches are operated in series circuits. Forexample, the select magnets H4, H6, etc., of all primary switches andthe select magnet I28 of the secondary switch pertaining to the N o. 0line link II8 are included in a series circuit. One end of this circuitis closed through contacts I32 to the positive pole of battery, and theother end of this circuit is closed through contacts I33 to the negativepole of battery. Likewise the select magnets of the primary switchesandsecondary switches pertaining to each of the remaining links areincluded in series circuits. All of the select magnet operating circuitsfor a horizontal group of links, such as the group I20, are connected inseries parallel relation through the contact I32 to the positive pole ofbattery. Similarly all oi the select magnet operating circuits incomingto any particular secondary switch, such as the switch IIO, areconnected in series parallel relation through the contact I33 to thenegative pole of battery. By closing the contact I32 at the primaryswitches and the contact I33 at the secondary switches a single circuitis completed through the windings of magnets I I4, II6, etc., of theprimary switches and the magnet I28 of the secondary switch IIO. In thismanner the select magnets of the primary switches and the secondaryswitch having access to the calling line and to an idle trunkrespectively are energized simultaneously to prepare these switches foroperation.

The hold magnets I34, I35, I36, I31, etc., of the primary switches serveto hold these switches in their operated condition after the selectmagnets have been released. Similarly the hold magnets I38, I39, etc.,of the secondary switch I I0 (and the same is true of the othersecondary switches) serve to hold the operated contacts in their closedcondition after the release of the select magnets.

The primary and secondary line switches disclosed herein may be of anywell-known type of crossbar construction, one suitable type beingdisclosed in the patent to Reynolds, No. 2,021,329 of November 19, 1935.

A general description will now be given of the circuits and equipmentshown in detail in Figs. 2 to 6. The No. 0 horizontal group of primaryswitches on the frame includes the No. 0 switch 200 and the No. 6 switch20I, there being. as above explained, five similar intermediate primaryswitches. Ten subscribers lines, including line 202 and 203, appear inthe vertical rows of contacts of switch 200 and are provided withindividual line discharge tubes, including the No. 0 and No. 9 tubes 204and 205. Similarly the No. 60 and No. 69 lines 206 and 20! appear in thelast switch 20I in the group and are provided with corresponding linetubes 208 and 206. There are, therefore, seventy subscribers linesappearing in the horizontal group and seventy individual line tubescomprising the group 2I0.

In like manner each 01' the remaining nine,

The ten horizontal groups of primary line switches are provided with agroup of ten space discharge tubes 305. one for each of the horizontalgroups. These tubes serve to select one of the horizontal groups forservice to the exclusion of other groups having calling lines therein.The tube 306, for example, is individual to the No. horizontal groupshown in Fig. 2 and when operated renders this group effective andexcludes the remainin groups. The other tube shown, tube 301, isindividual to the last' or No. 9 horizontal group shown in Fig. 3.

The tubes of each of the line groups 2| 0, 3|0, etc., are operatedselectively by means of multiplex signal impulses. These impulses areproduced by the impulse generator 308 which has seventy output circuits309. The generator 308 is energized by a source of alternating current 3of any suitable frequency and serves to produce momentary impulses ofseventy different phases during each successive cycle of the source 3|l. The impulses of the different phases, which are referred to herein asphase impulses, appear successively in the impulse circuits of the group309 and are applied to the control electrodes of the respective tubes ofthe group 2E0 and similarly to the electrodes of the respective tubes ineach of the other groups. For example, impulses of the No. 0 phaseappear in the conductor 3l-2, which is connected to the starting anode2!! of the No. 0 tube 204 and in multiple to the starting anodes of thecorresponding tubes in the other nine groups. Impulses of phase No. 9appear in impulse conductor 3|3, which is connected to the startinganode of theNo. 9 tube 205 in group 2l0 and to the corresponding tube inthe other groups. Impulses of phases No. 60 and No. 69 appearrespectively in impulse conductors 3l4 and 3|5, these conductors beingconnected to the starting anodes of tubes 208 and 209 and tubes 303 and304 and to corresponding tubes in other groups. When two or more linesin any oneof the horizontal groups initiate calls the line tubecorresponding to one of these lines operates depending upon the phase ofthe impulses at the time the calls are originated and excludes or looksout all of the other tubes in the group. Thus only one line tube in anyhorizontal group is permitted to operate at a time. This will beexplained more fully hereinafter.

The horizontal group tubes 305 are also operated selectively by phaseimpulse delivered by the generator 308. Since there are only ten ofthese horizontal group tubes only ten of the group of impulse generatorleads 309 are required. Any ten of the seventy leads may be useddepending upon the phases desired. As shown in the drawing, however, thefirst ten impulse conductors are connected to the starting electrodes ofthe ten tubes of the group 305. For example, the No. 0 impulse conductor3l2 is connected to the starting anode 3|6 and the No. 0 horizontalgroup tube 306, the No. 9 impulse conductor M3 is connected to thestarting anode 3|! of the No. 9

horizontal group tube 307, and the eight intermediate phase impulseconductors are connected to the corresponding intermediate tubes. of thegroup 305. When two or more. horizontal groups are calling at the sametime, one of the tubes 305 operates to prefer its horizontal group andto exclude the others.

The currents of the different phases may be obtained from the source 3in any suitable manner, and the momentary impulses delivered to thecircuits 309 for operating the tubes may also be produced by suitableimpulse generating means. For example, impulse generators designed toproduce momentary impulses of either positive or negative polarity aredisclosed in the, application of W. H. T. Holden, Serial No. 361,536,filed October 17, 1940; and W. H. T. Holden Patents 2,288,815, of June9, 1942; and 2,252,766, of August 19, 1941.

The ten links outgoing from the horizontal group shown in Fig. 2 extendrespectively to the ten secondary line switches. For example, the link212 appears in the secondary line switch 400, and link 2l3 appears inthe secondary line switch 500. The other eight links not shown in thisgroup appear in the corresponding secondary line switches. In likemanner the remaining groups of link circuits are distributed amongthesecondary line switches 400, 500, etc., the links 3| 8 and 3| 9 shown inthe last horizontal group appearing respectively in the secondaryswitches 400 and 500.

The No. 0 secondary line switch 400 has access to a group of ten trunks,including trunks 4M and 402, outgoing to some selective switching stagebeyond. The No. 9 secondary switch 500 has access to a similar group oftrunks which includes the trunks 50! and 502, and the same is true ofthe eight intermediate secondary switches. Thus the subscribers linesappearing in the ten different horizontal groups of primary switcheshave access through these switches and intermediate links and thesecondary switches to any one of ten groups of outgoing trunk circuits.

The secondary switches of the frame are provided with a common group oftesting tubes 600, there being one of these tubes for each of theoutgoing trunk groups. The first of these tubes, tube 60 l, isindividual to the group of trunks outgoing from the switch 400 andserves simultaneously to test these trunks for an idle one and to testthe link incoming to the switch 400 from the calling group of primaryswitches. The last tube of the group 600, tube 602, is individual to thegroup of trunks outgoing from the last secondary switch 500 and servessimultaneously to test these trunks and to test the link incoming to theswitch 500 from the calling group of primary switches. The matching testmade by the tubes of the group 600 is registered on the tubes of group605. There is one of these tubes in the group 605 for each of the tengroups of outgoing trunks, and the tube that is finally operated as aresult of the matching test by the group 600 indicates that there is anidle trunk in the associated group and that there is an idle link havingaccess to this trunk from the primary switches in which the calling lineappears.

The test of the trunk group is made by a circuit extending through thecontacts of the sleeve relays individual to the trunks of the group.Considering the trunk group appearing in the switch 400, the testcircuit for this group extends from the battery lead 403 through thenormal contacts in parallel of the sleeve relays 404, 405,.

of all idle trunks, thence through parallel resistances 406, 401,conductor 408, resistor 803 to battery and ground. If all trunks arebusy, this circuit is open, and no current flows through the resistor503. Therefore, no potential is applied through resistors 506 and 601 tothe starting anode 600 of the corresponding tube 609. However, if two ormore of the trunks are idle, the amount of current flowing throughresistor 603 results in theapplication of a potential of a predeterminedvalue to the anode of the tube 608. This potential alone, however, isnot sufilcient to ionize the control gap of the tube. Simultaneouslywith the foregoing test of the trunk group a test is also being made ofthe particular one of the ten links M2, M8, etc., which extends toswitch 400 from the calling group of primary switches. If the group ofFi 2 is calling, for example, the link 2 I2 is tested by means or acircuit extending over the conductor 2l4 through resistor 409, conductor4 I to one of the multiple starting anodes oi the tube 60L If the link2l2 is idle. positive potential appears on this anode, and the gapincluding the anode and the cathode H0 is ionized. If, however, the linkU2 is busy, the usual ground potential -on the sleeve con ductor H5 isapplied over conductor 0 to the starting anodeof the tube 6M, and thecontrol gap is not ionized. The test of the other nine groups of trunksand the simultaneous test of the respective links having access to thesetrunks are made in a similar manner by means of the corresponding tubesof the groups 600 and 605. For example, the last group of trunks,including trunks 5M and 502, are tested by means of a circuit extendingfrom the battery supply conductor 403 through the back contacts inparallel of the sleeve relays 503, 504, of all idle trunks,

discharges in a circuit traceable from the battery supply conductor 403.resistors 822 and 823, anode 824, cathode 8l0, resistor 828 to thenegative pole of battery 628. Assume also that a moment later an impulseof the corresponding phase is applied by the generator 615 to theimpulse conductor 821. This impulse passes through the discharged tubeGUI, and current flows through the resistor 025. This added flow ofcurrent .as a result or the impulse causes the application 01 anincreased positive potential through condenser 620, resistor 801 to thestarting anode 000 of the tube 009. This increased potential applied tothe anode 008 is sufiicient to ionize the control gap formed by theanode 608 and the starting cathode 029. Thereupon the tube 009discharges and signifies the selection of the associated trunk group andits matching idle link. Thev operated tube 009, as will be explainedmore fully hereinafter, also looks out the remaining tubes of the group605 to prevent the selection of another group of tnmks for serving thisparticular call. As the generator 6|! proceeds to transmit impulses ofsucceeding phases to the remaining conductors of the group BIB,corresponding impulses are applied to the cathodes of all tubes of thegroup 600 that are in a discharged condition because they representqualified trunk groups that are reached by idle links from the callingprimary group. However, these impulses are ineffective to cause theoperation of a second one oi! the tubes 60!, in case the associatedtrunk groups have idle trunks therein, for the reason stated, namelythat the operation of the first tube 609 to select the associated groupand link automatically renders the remaining tubes of the group 605ineffective.

Since the positive potential applied to anode 600 must be enough tooperate the tube it two trunks are idle and must not be enough tooperate the tube falsely if several trunks are idle,

the application of a potential of a predetermined value to the anode GIZof the tube 6l3. At the same time the link having access to this trunkgroup, such as the link 213, is tested by means of a circuit extendingover conductor 2 i 6, through resistor 508, conductor 509 to one of themultiple starting anodes of tube 602. If the link2i3 is idle, positivepotential appears on this anode and the gap including the anode and thecathode H4 is ionized.

As a result of these tests, therefore, each tube of the group 600corresponding to a trunk group which is reached by an idle link has itscontrol gap ionized; and each tube of the group 605 corresponding to atrunk group having two or more idle trunks therein has a positivepotential applied to its starting anode. Since one trunk group and itsmatching idle link should be selected to the exclusion of other groups,the operation of tubes 605 is controlled by phase impulses deliveredfrom the impulse generator Eli. The ten impulse conductors SIG to whichthe generator SIB delivers impulses of ten different phases, areconnected through condensers 0H, 8 to the main anodes of the ten tubesof the group 600. The first one of the tubes 600 to receive an impulsefrom the generator M5 after the foregoing test has been made causes theop eration of the corresponding tube of the group 605. Assume forinstance that the control gap oitube 60! is ionized because theassociated trung group contains two or more idle trunks. {is soon as thecontrol gap ionizes, the main gap the potential on conductor 408 is heldbelow a predetermined value by means of the battery 88!, which .has itspositive pole connected through the varistor 062 to conductor 400. Thus,no matter how many trunks are idle, the potential of anode 600 is heldto such a value that the phase impulse delivered to the tube 60! isneeded to operate tube 609, having in mind the positive potentialapplied to cathode 620 from battery 0" through resistor 664.

When all trunks are busy, battery ii! is disconnected from conductor408. To avoid the possibility, under this condition, of tube 608ionizing on the combined voltage of batteries 663 and 005 a varistor 606is connected between conductor 400 and ground.

It was mentioned above that the trunk groups are first tested todetermine whether they have two or more idle trunks. In case no trunkgroup is found having as many as two idle trunks a reserve test isthen'made to determine whether any group has a single idle trunk. Thistest is made by the tube 0 and the associated circuits. At the time thematching test is started battery potential is applied to conductor I20,and current fiows through the resistor "I to charge the condenser 032.After an interval or time has expired the condenser "2 reaches apredetermined charge, and the control gap 0! the tube 030 formed by theelectrodes 0" and 034 ionizesa Current now flows in the main dischargecircuit of the tube traceable from battery supply conductor 2|.conductor III. winding of relay 030,

anode 631, cathode 634 to battery and ground.

Relay 636 operates and applies direct ground potential to the startingcathodes 629, 638, etc., of the tubes of the group 605. With directground potential on the starting cathodes of these tubes the control gapof any tube will ionlze if a single trunk in the group is idle. In otherwords, a single idle trunk results in current flow over conductor 403through the closed contact of a single one of the sleeve relays, such asrelay 404, resistor 406, conductor 408, resistor 603 to battery. Theintensity of this current flow through resistor 603 causes theapplication of a potential to anode 608 which is somewhat less than thepotential applied thereto when two trunks are idle. However, the voltageproduced across the gap 608-629, when the generator 6! delivers itsphase impulse to the tube 60 I, and with electrode 629 at groundpotential, is suflicient to ionize the gap. Thus the reserve testresults in the selection of any group having a single idle trunkmatching with an idle link.

When one of the tubes 605 operates to indicate the selection of thetrunk group, an asso ciated relay 639, 640, operates to prepare for theselection of an idle one of the trunks in the of the common equipment aswill be described hereinafter. The condenser 4|3 discharges through therelatively low resistance 65! and cannot assume a sumcient charge tocause the group. Relay 630, for example, closes a circuit from negativepole of battery through its contacts, conductor 64!, resistor 435 to thestarting cathodes of the tubes of group 4I2, there being one of thesetubes for'each of the ten trunks 40!, 402. The startinganodes of thetubes 4! 2 are connected through the normal contacts of the sleeverelays 404, 405 to the ten phase impulse leads 642 outgoing from theimpulse generator 6l5. Therefore, as soon after one of the relays 639,640 operates as the generator 6l5 delivers an impulse of the phasecorresponding to the first idle trunk the corresponding tube 4!2 ionizesand prepares for the operation of the associated hold magnet of theswitch 400. The selection of an idle trunk in any one of the othergroups, such as, the group outgoing from the secondary switch 500,proceeds in the same manner under the control of the associated relay,such as the relay 640 individual to the tube 6! 3.

The impulse generator 6!5 may be similar to the generator 308; or, ifdesirable, the impulse leads 6!6 and 642 may be connected directly tothe output leads of generator 308 for obtaining pulses of the requiredphases for application to tubes 600, 605 and M2.

The common matching and testing mechanism shown in Fig. 6 is released,as soon as it has performed its functions, underthe control of a groupof release tubes 645. There are ten of these tubes 646, 641, etc., onefor each of the ten trunk groups. The starting anodes of these tubes areconnected through resistors and the front contacts of the sleeverelays'of the trunks to condensers. For example, the anodes of tube 646are connected through resistors 648, 649 through the contacts of relays404 and 405 to condensers M3 and M4. When the testing operationcommences, battery potential is applied to conductor 403. and condensersM3 and M4 assume a charged condition. Later when the trunk is selectedand the sleeve relay operates, the potential of the associated condenseris applied to the starting anode of the tube 646, and this tube ionizesits control gap. The main discharge gap of the tube 646 is then ionized,and current flows through the release relay 650. Relay 650 brings aboutthe release reionization of the tube 646 for the reason that theresistor 4! 5, through which the condenser charges, is considerablyhigher .in value than the resistor 6I5, through which it is constantlydischarging while the trunk 40! is in use. Therefore, other trunks inthe same group may be selected subsequently, and the tube 646 operatesas soon as each trunk is selected in turn to cause the release of thecommon mechanism.

A description will now be given of the detailed operation of the system,assuming for this purpose that the subscribers of lines 202 and 30!initiate calls at about the same time. When the subscriber of line 202removes his receiver from the switchhook, thus connecting a lowresistance across his line, an obvious circuit is closed through theback contacts of the hold magnet 2!! including the resistor 2!8. Thecurrent flowing through the resistor 2! 8 places a negative potential onthe starting cathode 2!!! of the associated tube 204, and this tubeionizes as soon as a positive impulse is delivered by the generator 308over conductor 3!2, resistor 220 to the starting anode 2| As soon as thetube ionizes, current flows from the positive pole of battery 6l9,conductor 62!, resistors 22! and 222, anode 223, cathode 224, winding ofthe hold magnet 2 I! to the negative pole of battery. The currentflowing in this circuit is not enough to cause the operation of the holdmagnet 2 1. The voltage drop, however, produced by the current flowingin resistors 22! and 222 lowers the potential of the anodes of theremaining tubes in the group 2!0 to a point where no other one of thesetubes can operate. Prior to the operation of the tube 204 the condenser32! associated with the horizontal group tube 306 is in a chargedcondition. The charging circuit for this condenser may be traced fromthe positive pole of battery over conductor 62!, resistor 22!, condenser32!, resistor 322 to ground. At the instant the tube 204 dischargessudden current flow through the resistor 62! lowers the positivepotential applied to the condenser 32!, resulting in the application ofa negative potential through the resistor 323 to the starting cathode324 of the tube 306. This negative potential is applied to the cathode324 until the condenser 32! has time to adjust its charge in the circuitincluding the resistors 322 and 22!. This interval is sufliciently longto permit the impulse generator 308 to apply a positive impulse overconductor 3! 2 and resistor 325 to the starting anode 3!6 of the tube306.

In a similar manner the initiation of a call on line 30! causes theoperation of the associated line tube 303. Tube 303 causes theapplication of a negative impulse to the starting cathode 33! of thehorizontal group tube 301, and this potential remains on the electrode33! sufiiciently long to permit the generator 308 to deliver a positiveimpulse of the corresponding phase to the starting anode 3!!.

Depending upon the phase of the generator 308 at the time the cathodes324 and 33! are rendered negative in potential, an impulse of positivepotential will be applied first to the anode 3!6 of the .tube 306 or tothe anode 3!! of the tube 301. Assuming that the phase is such that thenext impulse is delivered over the conductor 3!2 to the anode 3!6,thetube 306 ionizes its control gap 3I6-324, and current now flows fromthe positive pole of battery 610, conductor 62l,conductor 326, windingof the common relay 321, anode 328, cathode 323, group relay 330 to thenegative pole of battery. Relay 330 operates to indicate the selectionof the No. group of primary switches 200, 20l for service; and the flowof current through the common impedance 321 lowers the potential on theanodes of the remaining tubes of the group 305 to. a value such that noone of these tubes can operate. Therefore, the tube 301 representingcalling line 30| fails to operate when the generator 308 delivers thecorresponding impulse to the anode 3H, and the subscriber's line 30lmust wait until the other line has been served.

When the horizontal group relay 330 operates, circuits are prepared forthe select magnets associated with the ten link 2|2, 2l3 outgoing fromthe primary group serving the calling line 202 to each of the tensecondary switches 400, 500 on the frame. The preparation or theseselect magnet circuits consists in connecting the positive pole ofbattery to them. The circuit for the select magnets associated with thefirst link 2|2 may be traced from the positive pole of battery 6l9,conductor 62l, conductor 326, contacts of relay 330, conductor 332,through the winding of select magnet 225, thence over conductor 2| 4through the windings of the corresponding select magnets, includingmagnet 226, of the other six primary switches, thence through the selectmagnet 6 associated with the link 2l2 in the secondary switch 400 to thecommon conductor M1. The circuit for the select magnets of the link 2l3may be traced as before over the common conductors 62l, 326, contacts ofrelay 330, common conductor 332 thence through the windings of selectmagnets 221, 228 and over conductor 2l6 through the select magnet 5l0associated with the link 2|3 in the secondary switch 800 to the commonconductor 5| l. Likewise parallel circuits may be traced from thepositive pole of battery 6! 9 through the select magnets 01 all of theremaining eight links outgoing from the primary group to the remainingeight secondary switches not shown. These circuits, however, are not yeteffective to cause the operation of any of these select magnets sincethe common conductors 411, 5| l, etc., at the secondary switches areopen and will remain open until it is determined which trunk group is toserve the calling line 202.

When the common relay 321 operates, it also extends the positive pole ofbattery 6l9 by way of conductor 62! through the contacts of relay 321,conductor 333, conductor 403, thence through the back contacts of thesleeve relays of all idle trunks in all of the ten trunk groups outgoingfrom the secondary switches 400, 500. As hereinbefore explained, "theseparallel circuits through the back contacts of the sleeve relays extendthrough the resistors 606, 601 and 652, 653, etc., to the startinganodes of the test tubes 605. Battery potential on the conductor 333 isalso extended over conductor 4! to the main anode 624, 654, etc., of theten tubes 600. Each one of the tubes 600 representing a trunk group thatis reached by an idle link extending from the calling group is ionizedas above ex-' plained; and, when the generator 6l5 applies a positiveimpulse through the discharged tube 600 to the anode of the associatedtube 605, the latter tube operates to indicate the selection of a trunkgroup having an idle trunk therein and accessible to an idle link fromthe calling primary group. Assume that the tube 600 is the one operated,indicating that the trunk group including trunks 40l and 442 has beenselected, the operation of tube 600 results in the closure oi! a circuitfrom the positive pole 0! battery 6". conductor 62L conductor 636,impedance element 656, anode 651, cathode 656, winding of relay 639 tothe negative pole of battery.

When relay 630 operates it connects the negative pole of battery to theselect magnet operating .circuits of all ten links incoming to theswitch 400 .irom the ten different primary groups of switches. It willbe noted that only one oi these select magnet operating circuits hasbeen prepared by the connection of positive battery thereto, namely, theselect magnet operating circuit for the link 2l2 coming from the callingprimary group containing the subscribers line 202. Hence this one selectmagnet circuit is closed while the remaining circuits remaininefiective. This circuit may be traced from the negative pole ofbattery through the middle contacts of relay 633 to the conductor 4"thence as previously traced through magnet 4", conductor 2l4, magnets226, 226, conductor 332 to the positive pole of battery BIS. Similarlyrelay 639 partially completes parallel circuits for the remaining ninelinks incoming to the secondary switch 400. One of these may be tracedover the common conductor 1 through the winding of select magnet 4l9,conductor 420 through the select magnet 334 and other select magnets 01the primary switches to the common conductor 335. These circuits,however, are open at the primary switches, and none of them is fullyclosed. Primary select magnets 205, 226, etc., and secondary selectmagnet 4", however, are energized in the circuit above traced to preparefor the establishment of the connection over the selected link 2l2 toone 01 the ten trunks in the group outgoing from the switch 400.

The operation of relay 630 also prepares for the test of the ten trunksin the selected group to choose an idle one thereof for use. Relay 639at its inner contacts closes a circuit over conductor 64! through theresistor 435 to the starting cathodes of the tubes 412. Assume that thefirst trunk 40l in the group is idle and that the phase of the impulsegenerator H5 is such that the next impulse is delivered to theimpulseconductor 659. This impulse is applied through the resistor 42Ithrough the back contacts of the sleeve relay 404 01' the idle trunk40I, conductor 422 through resistor 423 to ground. This impulse causesthe application oi positive potential to the starting anode 424 01 thetube 426, which is individual to the trunk 40L Since the startingcathode 426 is at negative potential, the gap 424-426 ionizes when theimpulse is delivered, and the main discharge gap ionizes resulting inthe flow of current in the following circuit: positive pole of battery6l9, conductor 62!, resistor 421, anode 428, cathode 420, wind ing ofthe hold magnet 430 to the negative pole of battery. Current flowing inthis circuit alone is not sufllcient to energize the magnet 430.However, as soon as the select magnet 4| 6 has been operated as abovedescribed, a parallel circult is traceable from conductor 62| throughthe closed contacts 'of magnet 6, common impedance 43l thence throughthe tube 425 and the magnet 430 to battery. The magnet 430 operates inthis circuit and closes the contacts of the switch to complete theconnection from the incoming link 2l5 to the selected outgoing trunk 40LThe flow of current through the common impedance element 43l lowers thepotential of the anodes of the remaining tubes of the group 2 to preventanother one from operating in response to the succeeding phase impulsesdelivered thereto by the impulse generator 6l5.

At the time the primary select magnet 225 operates as above described aparallel circuit is closed from the battery supply conductor 62| throughthe closed contacts of magnet 225, common impedance element 229, anode223, cathode 224 of the tube 204 and winding of the hold magnet 2|! tobattery. The added current flowing through the parallel circuit justtraced causes the energization of the hold magnet 221. Magnet 221 closesthe contacts of the switch 200 to complete the connection from thecalling line 202 to the selected link 2l2'.

Thus the calling subscriber's line 202 is extended through the switch200 over the link 2|5 through the switch 400 to the idle trunk 40L Therelay 432 operates in an obvious circuit in series with the loop of thesubscriber's line 202 and closes an operating circuit for the sleeverelay 404. Relay 404 operates and applies ground potential to the sleeveconductor 433. The hold magnet 2|! of the primary switch is nowmaintained energized in a circuit traceable from battery through thewinding of the magnet contacts of the switch 200, sleeve conductor 2l5,contacts of switch 400 to the grounded conductor 433. The hold magnet430 of the secondary switch 400 ismaintained energized in a circuit frombattery through the windings of sa d magnet through the groundedconductor 433. The grounded conductor 433 is extended through contactsof the switch 400 thence over conductor to the start anode ofthe testtube 60l, thus identifying the link 215 as busy when the next test ismade.

The operation of the sleeve relay'404, indicating that the connectionhas been completed to the trunk '40l, causes the connection of thecharged condenser M3 to the start ng anode of the release tube 646. Thetube 646 discharges and release relay 650 operates in the main dischargecircuit thereof. Relay 650 operates the relay 620, which in turndisconnects the ma n battery supply from the conductor 62!. The operatedtubes and relays of the common control mechanism are now released torestore this mechanism to the common use of the switch frame. Therelease of'tube 306 and relay 330 reconditions the tubes 305 forsubsequent operation and causes the release of the operaed selectmagnets 225, 226 and M6. The release of tube 609 and relay 639 causesthe restoration of the tubes 605.

As soon as the horizontal group tubes 305 are released and the commonmechanism is restored to the common use of the frame, the callingsubscribers line 30! becomes effective in competition with other linesthat may be calling in the meantime to seize a horizontal group ofprimary switches and to initiate the testing operations above describedfor extending a call to an idle trunk in one of the groups.

It will be obvious that numerous modifications may be made in thedetails of the system herein disclosed. It may also be noted that thedischarge tubes illustrated may be of any suitable types such as thosecommonly known as gas filled cold cathode tubes.

What is claimed is:

1. In combination, primary switches, secondary switches, lines appearingin said primary switches, trunk groups appearing in said secondaryswitches, groups of links interconnecting the primary and secondaryswitches, operating magnets for said switches, a magnet operatingcircuit for each link including in series a primary switch magnet and asecondary switch magnet,-electronic devices for said primary switches,means for operating one of said devices to select a group of links forserving one of said lines, electronic devices for said trunk groups,means for operating one of said trunk group devices to select a group oftrunks to serve said line, means controlled by the operated electronicdevices for applying potential to the magnet circuits of the links ofthe selected group outgoing from the primaryswitches and to the magnetcircuits of the links incoming to the selected trunk group to efiect theenergization of the magnets of a single link interconnecting a primaryswitch in which said line appears and a secondary switch in which theselected trunk group appears, and means controlled by the energizationof said magnets for establishing a connection from said line over theselected link to an idle trunk in said group.

2. In combination, primary switches, secondary switches, lines appearingin said primary switches, trunk groups appearing in said secondaryswitches, groups of links interconnecting the primary and secondaryswitches, operating magnets for said switches, a magnet operatingcircuit for each link including in series a primary switch magnet and asecondary switch magnet,- a plurality of discharge tubes individualrespectively to said link groups, means for applying impulses'ofdifierent characters to said tubes for selectively operating one of themto determine the link group to serve a calling one of said lines, aplurality of discharge tubes individual respectively to said trunkgroups, means for applying mpulses of different characters to saidlatter tubes for selectively operating one which represents a, grouphaving an idle trunk and accessible to an idle link in the selectedgroup, means controlled by the operated tubes for applying potential tothe magnet circuits of all links of the selected group outgoing from theprimary switches and to the magnet circuits of all links incoming fromthe several primary groups to the second ary switch having access to theselected idle group of trunks to cause the energization of the magnetsof a single link interconnecting aprimary switch in which said callingline appears and a secondary switch in which the selected trunk groupappears, and means controlled by the energization of said magnets forestablishing a connection from said line over the selected link to anidle trunk in said group.

3. In combination, primary switches, secondary switches, lines appearingin said primary switches, trunk groups appearing in said secondaryswitches. groups of links interconnecting the primary and secondaryswitches, operating magnets for said switches, a magnet operatingcircuit foreach link including in series a primary switch magnet and asecondary switch magnet. a plurality of discharge tubes individualrespectively to said link groups. means for applying impulses ofdifierent phases to said tubes for selectively operating one of them todeterm ne the link group to serve a calling one of said lines, aplurality of discharge tubes individual respectively to said trunkgroups, means for applying impulses 01 different phases to said trunkgroup tubes for selectively operating one 01' said tubes representing agroup having an idle tmnkand accessible to an idle link in the selectedgroup or links, means controlled by the. operated tubes for applyingpotential to the magnet circuits of the links of the selected groupoutgoing from the primary switches and to the magnet circuits of thelinks incoming from the several primary groups to the secondary switchhaving access to the selected idle group of trunks to cause theenergization of the primary and secondary switch magnets of, a singlelink in the selected link group, and means controlled by the energizedmagnets for establishing a connection from the calling line to an idletrunk in the selected group.

4. In combination, primary switches, secondary switches, lines appearingin said primary switches, trunk groups appearing in said secondaryswitches, groups of links interconnecting the primary and secondaryswitches, operating magnets for said switches, a magnet operatingconductor for each link extending from a primary switch to a secondaryswitch, said conductor including in series a primary switch magnet and asecondary switch magnet, a plurality of electronic tubes servingrespectively said groups of links, means for operating one of said tubesselectively to choose the corresponding link group to serve one of saidlines, a plurality of electronic tubes serving respectively said groupsof trunks, means for operating selectively one oi? said trunk grouptubes representing a group having an idle trunk and accessible to anidle link in the selected link group, means controlled by the operatedlink group tube for connecting at the primary switches battery potentialto the magnet operating conductors 01' all links in the selected linkgroup, means controlled by the operated trunk group tube for connectingat the secondary switches battery potential to the magnet operatingconductors of all links incoming from the several link groups to thesecondary switch having access to the selected idle trunk group, therebycausing the energization oi the primary and secondary switch magnets fora single one or the links in the selected group, and means controlled bythe operation of said magnets for extending a connection to an idle oneof th trunks in the selected group. a

5. In combination, primary switches arranged in groups, secondaryswitches, each group of primary switches having a plurality of linksoutgoing therefrom, the links of each primary group appearing in allprimary switches of the group and appearing respectively in saidsecondary switches, each primary and secondary switch having anoperating magnet for each link -appearing therein, a magnet operatingcircuit for eachlink including in series the operating magnet of eachprimary switch and the operating magnet of the secondary switch, trunkgroups accessible respectively to said secondary switches, electronicdevices serving respectively said primary switch groups, means forselectively operating one of said primary group devices to select theassociated primary switch group for extending a connection, electronicdevices serving respectively said trunk groups, means for selectivelyoperating one of said trunk group devices to select a trunk group whichhas an idle trunk and which is reached through the secondary switch byan idle link in the selected primary group, means responsive to theoperated primary group electronic device for applying a potential to themagnet circuits of the links of the selected primary switch group, meansresponsive to the operated trunk group electronic device for applyingpotential to the magnet circuits of the links incoming to thecorresponding secondary switch from the primary groups for eflecting theoperation of the primary and secondary switch magnets for the singlelink extending from the selected primary group to the selected trunkgroup, and means controlled by the operated magnets for completing aconnection to an idle trunk in the selected group.

6. In combination, primary switches arranged in groups, lines appearingin said primary switches, secondary switches, each group of primaryswitches having a group of links outgoing to all of said secondaryswitches, trunk groups accessible respectively to said secondaryswitches, a plurality of line discharge tubes, means for operating saidtubes selectively by phase impulses to determine the particular callingline in a group or primary switches to be served, link group tubesindividual respectively to said groups of links, means for operating oneof said link group tubes selectively to determine the group oi primaryswitches to serve one of its calling lines to the exclusion of otherprimary switch groups,

trunk group tubes individual respectively to said groups of trunks,means for operating one of said trunk group tubes selectively todetermine the trunk group to serve the calling line, operating magnetsfor said primary and secondary switches, a magnet operating circuit foreach link including in series the operating magnet of each primaryswitch in which said link appears and the operating magnet of saidsecondary switch in which said link appears, means controlled by theoperated link group tube for applying potential to the magnet operatingcircuits of all links of the group selected to serve a calling line,means controlled by the operated trunk group tube for applying potentialto the magnet operating circuits of all links incoming to the secondaryswitch having access to the selected trunk group, thereby energizing anoperating magnet for a' single link in each oi. the primary switches inwhich said link appears and the operating magnet of the secondary switchin which said link appears, and means controlled by one of the operatedprimary switch magnets and by the operated secondary switch magnet forestablishing a connection from the calling line chosen for service overthe selected link to an idle trunk in the selected trunk group.

7. In combination, primary switches, secondary switches, lines appearingin said primary switches, trunk groups appearing in said sec ondaryswitches, groups or links interconnecting the primary and secondaryswitches, operating magnets for said switches, a magnet operatingcircuit for each link including in series a primary switch magnet and asecondary switch magnet, a plurality of discharge tubes individualrespectively to said link groups, means for applying impulses ofdifferent characters to said tubes for selectively operating one of themto determine the link group to serve a calling one of said lines, aplurality oi! discharge tubes individual respectively to said trunkgroups, means for applying impulses of difierent characters to saidlatter tubes for selectively operating one which represents a grouphaving an idle trunk and accessible to an idle link in the selectedgroup means controlled by the operated tubes for anplying potential tothe magnet circuits of all links of the selected group outgoing from theprimary switches and to the magnet circuits of all links incoming fromthe several primary groups to the secondary'switch having access to these ected idle group of trunks to cause the energization of the magnetsof a single link interconnecting a primary switch in which said lineappears and a secondary switch in which the selected trunk groupappears, means controlled by the energization of said magnets forestablishing a connection from said line over the condition.

LARNED A. MEACHAM. CARLYLE V. PARKER.

